What Is An Ion In Chemistry?

by Kate Onissiphorou

An ion is an atom that has an electric charge because it’s either lost or gained one or more of its electrons. In chemistry, positively-charged ions are called cations and negatively-charged ions are called anions. 

Ions can exist in solutions or on their own depending on specific conditions, such as the presence of ionising radiation or a strong electrical charge like a lightning strike. 

Ions play a crucial role in nature, especially in biological systems. As well as helping to maintain the structure of cells, they’re involved in many cellular processes, such as transporting nutrients and expelling waste products. In multicellular organisms such as humans, ions are crucial in transmitting signals from the brain to other organs. 

Ions are important in chemistry experiments and industrial production, too. For example, they’re used to manufacture different types of batteries and prepare chemical products, such as extracting hydrogen and oxygen from water through electrolysis. Ions are also involved in the electroplating process, which relies on non-spontaneous redox reactions. 

Put simply: what is an ion?

The simplest definition of an ion is that it’s a charged atom or group of atoms. It can either be positively or negatively charged, depending on how many electrons it has. A graphic showing the atomic structure

Once an atom loses one or more of its valence electrons, it becomes positively charged (this is called a cation). Conversely, once an atom gains one or more electrons, it becomes negatively charged (an anion). You can calculate an atom’s charge based on the number of electrons that are either lost or gained.

Many substances form ions in aqueous solutions, but this is especially true for salts, acids, and bases. Their ions dissociate in water when dissolved. However, weak acids and weak bases do not completely dissociate in water. Even pure water partly dissociates into hydronium ions and hydroxyl ions (a process known as self-ionisation).

What do ions do? 

Ions are charged atoms that are usually formed by substances with ionic bonds such as salts and minerals. An ionic bond, also known as an electrovalent bond, is the strongest type of chemical bond as the electrons are not shared equally between the atomic constituents of a substance. Rather, the valence electrons of one atom are permanently transferred to another atom in the same substance.

As well as being good electrical conductors, ions can store charges if they’re arranged in a certain way. Batteries, for instance, rely on ions when storing electrical energy. Lead and LiFePO batteries are composed of multiple galvanic cells that have electrolytes and electrodes. The galvanic cell can be made of any material that has different types of electrodes and electrolytes. For example, the typical dry-cell battery has carbon and zinc electrodes.

Close up of rows of batteries

The electrical charge is stored in the ions that are dissolved in the electrolyte. In a battery, the electrons flow from the cathode to the anode. Spontaneous redox or oxidation-reduction reactions occur in batteries when the two electrodes are connected to a load or are allowed to short-circuit.

Ions also play a vital role in biological systems and the structural and physiological functions of organisms. For example, calcium and sodium ions are important in transmitting synaptic signals from and into the brain.

Along with other neurotransmitters, calcium and sodium ions make it possible for our brains to communicate and control various parts of the body. These include involuntary actions like breathing, as well as voluntary movements such as walking. The brain receives sensory information from different parts of the body through electrochemical signals that are facilitated by ions.

Negative vs positive ions

Negative ions, or anions, are atoms that have gained electrons. The excess electrons make them negatively charged. Meanwhile, positive ions are atoms that have lost electrons, thus making them positively charged.

Whether an atom is negative or positive in a compound that’s dissolved in water depends on its electronegativity. As the electronegativity increases, an atom in a compound is likely to become negatively charged. Conversely, an atom that has relatively low electronegativity tends to be positively charged.

This is related to an atom’s tendency to either be reduced or oxidised during a chemical reaction. As you can see in the periodic table of elements below, fluorine is a very strong oxidising agent because it has a high affinity for electrons. 

Conversely, the alkali metals in Group 1A have a higher tendency to lose electrons. These elements therefore partner well with elements with high electronegativity, resulting in the formation of ionic bonds. Periodic table of elements illustrating the electronegativity trend

How do we form negative ions?

As shown in the previous illustration, some atoms or elements have a greater tendency to accept electrons in a chemical reaction. These atoms form negative ions when they dissociate from their positively-charged counterparts in an aqueous solution. Negative ions can also be formed by bombarding atoms with electrons.

How are positive ions formed?

In compounds with ionic bonds, or in acids or bases, the metallic constituents tend to form positive ions when they dissociate in water solutions. In an ionic bond, they give away their electrons to their partner atoms that have high electronegativity. 

Positive ions can also be formed by bombarding atoms with ionising radiation such as ultraviolet light to dislodge the valence electrons. This is the principle of the photoelectric effect.

What is the difference between an ion & an atom?

The main difference between an ion and an atom is the net charge. An ion can either have a positive or negative net charge, while an atom always has a zero net charge. 

This is because an ion is an atom that either loses or gains electrons. When an atom gains electrons, it becomes negatively charged. Conversely, when an atom loses electrons, it becomes positively charged.


In chemistry, an ion is an atom that has either lost or gained electrons. That means it can either be positively charged (in this case it’s called a cation) or negatively charged (an anion). Ions form ionic bonds with corresponding ions. An atom’s electronegativity can influence whether it has a tendency to donate or accept electrons. Ions play a vital role in many types of chemical, industrial and biological processes. 


The blog on chemicals.co.uk and everything published on it is provided as an information resource only. The blog, its authors and affiliates accept no responsibility for any accident, injury or damage caused in part or directly from following the information provided on this website. We do not recommend using any chemical without first consulting the Material Safety Data Sheet which can be obtained from the manufacturer and following the safety advice and precautions on the product label. If you are in any doubt about health and safety issues please consult the Health & Safety Executive (HSE).